This invention is related to the art of treating aluminum parts to detect visually those localized areas which have been degraded by improper processing. Such areas can be found in heat treated aluminum alloy products as a result of variations in quenching from the solution treating temperature, overaging of nonuniform precipitates or by overheating during subsequent processing. What actually results is that local areas in a particular part have lower hardness and mechanical properties than other areas of the part. Parts with such conditions must be detected so that an aircraft or other device is not built from material with inferior mechanical properties.
These local areas of hardness or strength differences can be detected if each square inch of a component is inspected by standard methods such as hardness or electrical conductivity testing. But such extensive testing is uneconomical so a visual detection method is preferred. One known method for visually checking the entire surface of an aluminum part is by anodizing. In an anodizing process, an electric current is used with the part being one of the terminals to produce a coating on the aluminum. Such a surface treatment reveals improperly processed areas as differing in color from the properly hardened areas.
Using an anodizing process to detect these areas of hardness differences has several disadvantages. Among these are the high cost of anodizing, and the long processing time involved. Also, a particularly significant problem is the loss of metal resulting from removing the anodized coating after inspection. This sometimes results in dimensional changes on close tolerance parts to such an extent that the parts become nonacceptable. Another disadvantage of anodizing is that on nonmachined parts, such as forged or rolled surfaces, the anodizing process sometimes produces erratic results, resulting in missing the detection of localized variations in strength and hardness.
Among the other advantages of the present invention is that the cost to detect local areas of strength and hardness difference in aluminum parts is reduced by two-thirds or more, and the time is reduced from about 40 minutes to 10 minutes or less. The metal loss is minimal and the film can be stripped in 10-15 seconds after inspection.
A further advantage of the new process is that the film does not interfere with penetrant inspection, whereas an anodic film must be removed before penetrant inspection. Thus, a processing step is eliminated because the film can be easily removed during the normal cleaning required to remove the penetrant. In penetrant inspection, a part is checked for cracks or flaws by applying a composition which contains a visible dye or fluorescent material and is flowed into the flaws. Typical patents showing this type testing are U.S. Pat. No. 3,436,959 and U.S. Pat. No. 3,418,078.
This invention, unlike anodizing, gives a high degree of sensitivity and reliability making it possible to detect degraded areas in machined or forged/rolled articles.
Accordingly, a principal objective of the present invention is to provide a process for detecting improperly processed areas in aluminum without going through the anodizing technique. Another objective of the present invention is to provide a process for detecting local hardness/strength variations in aluminum parts by applying a solution containing an oxidizing agent and an acid or base to produce a visually perceivable color difference between properly and improperly processed areas of the aluminum. These and other objectives and advantages will become apparent hereinafter.